Distinct Functional and Pharmacological Properties of Tonic and Quantal Inhibitory Postsynaptic Currents Mediated by -Aminobutyric Acid A Receptors in Hippocampal Neurons DONGLIN BAI, GUOYUN ZHU, PETER PENNEFATHER, MICHAEL F. JACKSON, JOHN F. MACDONALD, and BEVERLEY A. ORSER Departments of Physiology (D.B., G.Z., M.F.J., J.F.M., B.A.O.) and Pharmaceutical Sciences (P.P.), University of Toronto, Toronto, Ontario, Canada; and Department of Anesthesia, Sunnybrook and Women’s Health Science Centre, Toronto, Ontario, Canada (B.A.O.) Received August 9, 2000; accepted December 22, 2000 This paper is available online at http://molpharm.aspetjournals.org ABSTRACT -Aminobutyric acid (GABA), the principal inhibitory neuro- transmitter, activates a persistent low amplitude tonic current in several brain regions in addition to conventional synaptic cur- rents. Here we demonstrate that GABA A receptors mediating the tonic current in hippocampal neurons exhibit functional and pharmacological properties different from those of quantal syn- aptic currents. Patch-clamp techniques were used to charac- terize miniature inhibitory postsynaptic currents (mIPSCs) and the tonic GABAergic current recorded in CA1 pyramidal neu- rons in rat hippocampal slices and in dissociated neurons grown in culture. The competitive GABA A receptor antagonists, bicuculline and picrotoxin, blocked both the mIPSCs and the tonic current. In contrast, mIPSCs but not the tonic current were inhibited by gabazine (SR-95531). Coapplication experi- ments and computer simulations revealed that gabazine bound to the receptors responsible for the tonic current but did not prevent channel activation. However, gabazine competitively inhibited bicuculline blockade. The unitary conductance of the GABA A receptors underlying the tonic current (6 pS) was less than the main conductance of channels activated during quan- tal synaptic transmission (15–30 pS). Furthermore, com- pounds that potentiate GABA A receptor function including the benzodiazepine, midazolam, and anesthetic, propofol, pro- longed the duration of mIPSCs and increased tonic current amplitude in cultured neurons to different extents. Clinically- relevant concentrations of midazolam and propofol caused a greater increase in tonic current compared with mIPSCs, as measured by total charge transfer. In summary, the receptors underlying the tonic current are functionally and pharmacolog- ically distinct from quantally activated synaptic receptors and these receptors represent a novel target for neurodepressive drugs. -Aminobutyric acid (GABA), the major inhibitory neuro- transmitter in the central nervous system, modifies electrical activity in the brain by regulating membrane hyperpolariza- tion and the “shunting” of excitatory input. GABA released from presynaptic terminal binds to GABA A receptors clus- tered at the postsynaptic membrane and activates inhibitory postsynaptic currents (IPSCs). In addition to conventional quantal synaptic transmission, a persistent form of GABAer- gic inhibition has been described in several brain regions. A small but significant tonic GABAergic current has been ob- served in the cerebellum (Brickley et al., 1996; Wall and Usowicz 1997), cortex (Salin and Prince, 1996), thalamus (Liu et al., 1995), and hippocampus (Otis et al., 1991). This tonic current has been best characterized in the cerebellum, where glomerular structures that surround synapses onto granule cells serve as a repository for transmitter released from neighboring synapses. Transmitter in the glomerulus may activate high-affinity GABA A receptors with minimal desensitization properties that are located in perisomatic and extrasynaptic regions of granule cells (Rossi and Hamann, 1998). The mechanisms that regulate the tonic GABAergic inhi- bition in other brain regions are not well understood. The tonic conductance in the hippocampus may result from the summation of overlapping miniature IPSCs (Soltesz et al., 1995; Salin and Prince, 1996), or the spill-over of vesicular transmitter released from neighboring synapses (Brickley et al., 1996; Rossi and Hamann, 1998). Recently, it was postu- lated that the tonic current results from the release of GABA from a surface matrix reservoir that becomes exposed during exocytosis (Vautrin et al., 2000). Also, reverse operation of GABA cotransporters (Gaspary et al., 1998) or release of GABA from astrocytes (Liu et al., 2000) might elevate GABA to concentrations sufficient to activate receptors. The in vivo ambient concentration of GABA in the extracellular space, measured using microdialysis (0.8 –2.9 M), is sufficient to ABBREVIATIONS: GABA, -aminobutyric acid; IPSC, inhibitory postsynaptic currents; mIPSC, miniature inhibitory postsynaptic current; aCSF, artificial cerebrospinal fluid; TTX, tetrodotoxin. 0026-895X/01/5904-814 –824$3.00 MOLECULAR PHARMACOLOGY Vol. 59, No. 4 Copyright © 2001 The American Society for Pharmacology and Experimental Therapeutics 479/891373 Mol Pharmacol 59:814–824, 2001 Printed in U.S.A. 814 at ASPET Journals on May 27, 2017 molpharm.aspetjournals.org Downloaded from